1. Field of the Invention
The present invention relates generally to crude oil processing. In another aspect, the present invention relates to a method and apparatus for heavy oil extraction. In another aspect, the present invention relates to a method and apparatus for preparing and treating an extraction solvent.
2. Description of the Prior Art
Heavy oil extraction processes (otherwise known as solvent de-asphalting) have been used, for example, in the production of heavy lubricating oil fractions, hydrocarbon cracking feedstocks, and low sulfur fuel oil feedstocks. Heavy oil extraction typically utilizes light hydrocarbon solvents which selectively remove useful extracts from heavy oil crude fractions at low temperature. The extracts obtained are rich in paraffinic components and are relatively free of asphaltenes, metals, and high boiling aromatics. Following extraction, an asphalt bottoms product remains which can be used, for example, in the production of asphalt cement.
In the typical heavy oil extraction process, a heavy oil feedstock is contacted with a light hydrocarbon solvent in a countercurrent extraction unit. The heavy oil feedstock normally comprises reduced crude (crude residua) or vacuum reduced crude (vacuum residua) obtained from a crude fractionation unit. Suitable light hydrocarbon extraction solvents have been obtained from crude oil fractions by distillation, sulfur dioxide extraction, hydrogenation, and hydroforming.
Substantial heavy oil extraction unit operating costs are generated in providing a suitable extraction solvent and maintaining extraction solvent purity. In order to maintain sufficient extraction solvent purity, a slip stream of dirty recirculating solvent is withdrawn from the extraction process, as necessary, and fresh extraction solvent is added. The dirty solvent withdrawn from the extraction unit is typically combined with the extract product. Since the dollar value of the extraction solvent exceeds the dollar value of the extract product, the need to dump dirty extraction solvent into the extract product and add fresh make-up solvent creates a large unit operating expense.
The present invention provides a novel and economical method and apparatus for supplying a high purity extraction solvent and for maintaining solvent purity. The present invention combines the dirty solvent withdrawn from the extraction unit with a fresh solvent fraction obtained from a crude fractionation unit. The fresh solvent fraction and the dirty solvent are fractionated to provide a purified solvent make-up stream.
In addition to providing a high purity solvent and maintaining solvent purity, the present invention provides efficient and economical reclamation of used solvent; allows less rigorous solvent separations in the crude unit and the extraction unit; eliminates the need to buy high purity solvents for solvent make-up; and provides efficient and economical control of fresh solvent make-up.